A quick share on what I've done recently in a "media arts" themed Options class (like an elective for Junior high kids).
I found some pictures on the internet that displayed an obvious emotion - a girl running through a field, a man sitting lonely on a bench, a goal being scored in soccer, etc. Using Garage Band I had the students create a 30 second "theme song" to go with the picture - the criteria was "if someone heard your song without the picture, could they describe the same emotion as when they see the shot?".
Some lessons learned:
Any loop software works for this - Aviary was a close second choice in terms of using their instruments to create, but they don't have "loops" like Apple does.
Showing students some "good" music/clips can be really helpful - especially pointing out which instruments are generally used for which emotions.
Talking about beats per minute (here's a great math lesson!) can also help get students into the idea of creating music for a mood.
"Less is more" tends to be good for my kids - they want to put 6 tracks of loops all at the same time, completely drowning out the idea. Three or four max would be better.
Showing them how to "space" their tracks so that instruments and beats come in and fade out at different times can be an easy way for them to discover "creative" song production.
Try it yourself! You'll have a better understanding of some of the struggles the kids are about to go through if you've tried to create a mood using some sort of audio program. Some things I found were the difficulties in not being too repetitive, as well as the challenge of finding instruments to match specific moods.
Overall, it was a fun class, and the kids came up with some really neat songs to go with the pictures.